Earlier this year, Mr. Gallagher was mugged on his way home from a shift at Bar Tabac on Smith Street, where he worked as a waiter. "I turn around and this guy's got a two-foot machete in my face," he said.Mr. Gallagher was unhurt and the mugger was later caught by the police, but one night soon after the mugging, with the image of his attacker's dark silhouette still burned into his memory, Mr. Gallagher was mesmerized by a shadow on the sidewalk. He reached into his pocket and felt the chalk he had used to write the outdoor menu at Bar Tabac, and he dropped to his knees to outline it.Shadow art was born.